1. Field of the Invention
The present invention relates generally to image processing devices for outputting image signals to form images, and more particularly to automatic density correction for forming a distinct image.
2. Description of the Background Art
As means for reading images on digital copiers or facsimile devices, an image reading device has conventionally been employed which optically reads an original including a still image by an image sensor and then provides various image processings for read image data to output an image signal. The image signal outputted is utilized for image formation as a hard copy onto paper, or onto a liquid crystal display or CRT.
In such an image reading device, density correction for correcting image data to an optimal value is carried out in accordance with a shade of an image formed of characters or figures on the original, in order to form a distinct image. For example, even with a lighter shade image as drawn by pencils, image data is corrected so that an image formed as a hard copy, for example, has a suitable darkness.
Conventionally, density correction has been carried out by varying a reference voltage for analog/digital (A/D) conversion at the stage of A/D converting a read signal from an image sensor to generate image data of predetermined bits.
The amount of density correction is designated by operators, or alternatively, automatically adjusted. In the automatic density correction, the density of an image is detected in advance such as by preliminary scanning.
In a conventional reading device, when an original is a blue printed drawing or newspaper, where an image is formed on its colored background (except for white), the density of the background is corrected as well as that of the image formed thereon.
Thus, the conventional image reading device has presented such disadvantages as follows. In case of reading a lighter shade image formed on a non-white background to form an image, performing density correction so as to form a dark image results in formation of an indistinct image, which is an image as if it is formed on the stained background. On the other hand, when the density is corrected so that the background is not colored, the image itself is formed with a lighter shade and thus with a soft contrast.
In addition, since the conventional density correction entails analog control, not only accuracy in correction is unstable, but also characteristics of image processing such as tone reproduction and edge emphasis are liable to be deteriorated by a varied dynamic range of A/D conversion due to the density correction.